Solid - State Lasers

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The laser, by definition, is a device that amplifies light by means of stimulated emission of radiation. The major properties of laser radiation are high intensity, narrow width, directionality and coherence.

In practice a laser is generally used as a source or generator of radiation.

The working element of the ruby laser is a cylinder of pink ruby containing 0.05 per cent chromium. The cylinder is usually between 0.1 to 2 cm in diameter and 2 to 23 cm long; the end faces are plane and parallel to a high degree of accuracy (Fig.1).

In the commonly used laser configuration a ruby rod is surrounded by the coils of a helical flashlamp operated usually for a few milliseconds with an input energy of 1000 to 2000 joules.

Lasers require some type of resonator for the radiation field. A resonator provides for a stronger coupling between the radiation and the excited atoms.

flashtube

trigger ruby

electrode

Fig. 1. Construction of ruby laser.

The resonator most commonly used for laser action is composed of two small mirrors facing each other. When the ruby crystal is illuminated by short, intense bursts of white light from a flash tube, a red light beam of enormous power starts to bounce back and forth between the mirrors increasing in strength each time it passes through the ruby. One of the mirrors is partially transparent and from this mirror emerges the intense coherent radiation.

The intensity of this radiation exceeds that of the spontaneous radiation by several orders of magnitude, and spectral range of the induced radiation is considerably narrower than that of fluorescence. The narrowing of the line-width is due to effect of the resonant cavity formed by the mirrors.

Solid-state lasers generally operate in the pulsed condition. The reasons for this are mostly technical. First it is difficult to provide a powerful source of exciting light capable of continuous operation; second, a great deal of heat evolves within the laser which must be dissipated. Ordinary ruby lasers are excited for periods of a few milliseconds, the length of the period being determined by the duration of the exciting flash.

Semiconductor Lasers

Several fundamental modifications of the basic p-n junction electroluminiscent diode exist, and chief among these is the semiconductor injection laser. The gallium arsenide semiconductor laser was invented in 1962. (Fig.1).

Fig. 2. Construction of typical GaAs semiconductor injection laser.

The gallium arsenide diode consists of a layer of the p-type gallium arsenide and a layer of n-type gallium arsenide. In its simplest form, the injection laser is a direct band-gap LED having an exceptionally flat and uniform junction (the active region) bounded on facing sides by two parallel mirrors perpendicular to the plane of the junction, which provide a Fabry-Perot resonant cavity to produce quasi-coherent laser radiation.

The lasers usually operate at 71 K, or lower. The p-n junction is usually made by diffusing Zn (acceptor) in one side of a Te (donor)-doped GaAs crystal. The entire area of the junctions is of the order of 10 ^{– 4}cm².

When an intense electric current, about 20,000 amperes per square centimeter, is applied to the device, it emits coherent or incoherent light, depending on the diode type, from the junction between the two layers of gallium arsenide.

Induced recombination of holes and electrons produces photons, which stimulate in-phase recombination and photon emission by other holes and electrons.

The mirrors on either end of the active region provide the optical feedback necessary to sustain laser action, and a small fraction of the wave propagating between the mirrors emerges from each on each pass. One end face on many commercial lasers is overcoated with a reflective Au film to cause all the radiation to emerge from only one end of the device and thus enhance radiation efficiency.

The most common and best-developed injection laser utilizes GaAs (905 nm), though many other semiconductors have been used to produce wavelengths ranging from 630 nm (Al_xGa_yAs) to 34 µm (PbSnSe).

The development of a semiconductor laser is one of the most important developments in the rapidly changing field of technology. The advantages of this type of laser - a gallium arsenide (GaAs) diode - are great compared to crystal and gas lasers. Semiconductor lasers approach efficiencies of 100% as compared to a few percent of other types; they are excited directly by electric current while other lasers require bulky optical pumping apparatus; because they are excited by an electric current they can be easily modulated by simply varying the excitation current.

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